In recent years, researchers and engineers have been widely studying the creation of materials that significantly improve the function and property and that exhibit new function and property. For this purpose, they exploit the nanotechnology that controls the process for synthesizing particles at a nanometer (10−9 m) level. In the case of metal particles, it has been widely known that when the particle size is reduced, the dependency of the physical property of the particles changes from the volume dependency to the surface-area dependency, thereby changing the catalytic activity, the sintering temperature (which is reduced), and other physical properties. When the particle diameter of metal particles is reduced to below submicron, i.e., to a nanometer level, unique functions that cannot be obtained in a bulk state are manifested in properties, such as not only the above-described catalytic activity and sintering temperature (which is reduced) but also magnetic property, optical property, detection of specific genes, and coloring. Consequently, nanometer-size metal particles attract particular attention as a functional material.
However, as the particle diameter of metal particles is decreased to a nanometer size, the van der waals force between the metal particles increases, aggravating the problem of the intensification of the particle agglomeration. To cope with the agglomeration, a measure is usually employed in which the surface of the particles is coated with a dispersant that suppresses the agglomeration. Nevertheless, when the diameter of the particles is decreased to a nanometer size, the agglomerative force becomes excessively strong, creating a problem in that the redispersion becomes difficult. Furthermore, when the distance between the metal particles is increased to prevent the agglomeration, although the agglomeration can be decreased without failure, other problems are created in the handling such as weighing, transferring, and packing.
As a result, fine metal particles having a nanometer-size particle diameter have been usually supplied in the form of a colloidal dispersion solution. Examples of this technique have been disclosed in the published Japanese patent applications Tokukaihei 11-80647 and Tokukai 2001-35255.
On the other hand, when metal particles are combined with other material such as resin and adhesive, it is generally desirable that the metal particles be used in a dry state because of the following reasons: first, the control of the adding amount is easy and, second, when a dispersion medium such as water or organic solvent is used, it may adversely affect the composite material using resin, adhesive, and the like. Consequently, a dry powder superior in handling and redispersion has been awaited.